Split `webcodecs` into `audio-codecs` and `video-codecs`

[caugner]

I suggest to split up the webcodecs feature into two features audio-codecs and video-codecs, because they have different levels of browser support:

• AudioDecoder is available in all browsers except Firefox Android and Safari.
• VideoDecoder is available in all browsers except Firefox Android.

See: https://github.com/web-platform-dx/web-features/blob/main/features/webcodecs.yml.dist

/cc @ddbeck

[foolip]

I agree we should make this split. Ideally, the webcodecs feature would remain as a sort of composite feature, but we don't have the tools for that yet.

I think this should wait for #91 so that we can redirect to the two new features, however.